The present invention relates to a method for detecting the formation o colloidal species in solutions and in particular to a method for detecting the formation of sulfur-containing colloidal species within an aqueous solution which comprises polysulfide ions, and which may also comprise sulfide ions and/or hydroxide ions and/or sulfur.
U.S. Pat. No. 4,485,154 discloses an electrically rechargeable anionically active reduction-oxidation electricity storage/supply system and process using a sulfide/polysulfide anolyte reaction in one half of the cell and a halide/halogen catholyte reaction in the other half of the cell. The catholyte reaction is:
Hal2+2exe2x88x92=2Halxe2x88x92
On discharging the system the reaction moves from left to right and on charging the system the reaction moves from right to left. The catholyte reaction is:
S2xe2x88x92=S+2exe2x88x92
On discharging the system the reaction moves from left to right and on charging the system the reaction moves from right to left.
When the system is fully charged the sulfur in the anolyte is present as sulfide ions. As the system discharges elemental sulfur is produced which then dissolves in the anolyte solution by combination with sulfide ions to form polysulfide species such as S22xe2x88x92, S32xe2x88x92, S42xe2x88x92 and S52xe2x88x92. However, at a certain point in the discharge cycle there will no longer be sufficient sulfide ions present to solubilize the elemental sulfur as a polysulfide and consequently the elemental sulfur precipitates out of solution. In a solution of Na2S this would be expected to occur when the ratio of S/Na exceeds approximately 2.5. When the ratio is equal to 2.5 the elemental sulphur is solubilized as Na2S5, however, when the ratio exceeds 2.5 the elemental sulphur can no longer be solubilized as a polysulfide and consequently precipitates but of solution. It should be, noted however that the equilibrium between sulfur and aqueous polysulfides is strongly dependent upon the alkalinity of the solution. Longer polysulfide chains may be formed in alkaline solutions thus delaying the onset of precipitation until a higher ratio of S/Na is reached.
The formation of a precipitate of sulfur within the anolyte is undesirable because it may be deposited on the electrode reducing its conductivity and reducing the overall performance of the system. Thus it would be desirable to provide a method for detecting the onset of precipitation of sulfur from the anolyte so that the system may be switched over to the charge cycle before sulfur precipitates in the anolyte.
It is known that sulfur forms a colloidal species immediately prior to forming a precipitate. The nature of the sulfur colloidal species is discussed by R. Steudel, T. Gobel and G. Holdt in Z. Naturforsh. 43b, 203-218 (1987) and Z. Naturforsh. 44b, 526-530 (1989). The sulfur colloid is known to have a charged xe2x80x9cmicelle-likexe2x80x9d structure. If the formation of the colloidal species could be detected then this would provide a warning signal that the precipitation of sulfur is imminent and that the system should be switched over to the charge cycle to prevent precipitation occurring.
It is also known that charged particles, such as the charged xe2x80x9cmicelle-likexe2x80x9d structures of colloidal sulfur, can be detected by the technique of acoustophoresis. The principle of the technique is as follows. If an electric field is applied across a charged colloidal particle it will move in that field. In an oscillating electric field the motion of the particle will be proportional to the magnitude and frequency of the field. If a high frequency field is applied, and the particles respond, then high frequency motion will result. In acoustophoresis is the applied frequency is typically 106 Hz. Particle motion at this frequency generates a mechanical pressure wave with a magnitude characteristic of the mobility of the particle, its concentration and the density of the overall composition containing the particle. This is an acoustic wave travelling at the velocity of sound in the medium and the amplitude of the signal is called the Electrokinetic Sonic Amplitude (ESA). The ESA signal can be monitored as the overall composition changes, a sharp change in the ESA signal may be indicative of the creation of a new charged species within the composition. A review of electroacoustic phenomena has been made by Babchin et al.(Babchin, A. J.; Chow, R. S.; Sawatzky, R. P.; xe2x80x9cElectrokinetic Measurements by Electroacoustical Methodsxe2x80x9d, Advances in Colloid and Interface Science, 1989, Vol. 30, No. 1-2, pp. 111-151).
Accordingly, the present invention provides a method of detecting the onset of colloid formation within a solution whose composition is in a state of change, which method comprises the steps of either
(i) applying an oscillating electric field to the solution and monitoring the amplitude of the resultant acoustic signal, the onset of colloid formation being detected by a change in the amplitude of the resultant acoustic signal, or
(ii) applying an oscillating acoustic signal to the solution and monitoring the resultant oscillating electric field, the onset of colloid formation being detected by a change in the amplitude of the resultant oscillating electric field, or
(iii) applying an oscillating electric field to the solution and monitoring the resultant oscillating electric field, the onset of colloid formation being detected by a change in the amplitude of the resultant oscillating electric field.
Preferably the method uses step (i) of the three options listed above.